Zusammenfassung / Abstract

The focus of this thesis is on the properties of foams stabilized by aqueous solutions containing nonionic surfactants, namely n-dodecyl-b-D-maltoside (b-C12G2) and dodecylhexaethyleneglycolether (C12E6) as well as their 1:1 mixture. A three-dimensional foam is a complex structure which decays due to three main mechanisms. To understand the stability of such a complex system one can analyse, amongst others, the foam film as the smallest unit of the foam. In general a foam can only be generated if surfactant molecules adsorb at the water/air-interface. This means that in addition to the analysis of the foam films also the water/air-interface needs to be studied. For that purpose, the adsorption of the surfactants or the rheological properties of the surfactant monolayer can be investigated. In other words, these properties of freestanding foam films and the water/air-interface contribute to the understanding of foam properties.

The first part of the thesis is about the properties of foams stabilized by b-C12G2, C12E6 as well as by their 1:1 mixture, which are studied with the commercially available FoamScan (Teclis). The depletion of the surfactant solution during foaming has been determined quantitatively both theoretically and experimentally. Furthermore a protocol for studying and comparing aqueous foams has been established. This protocol leads to foams with very similar structures and therefore allows one to compare the properties of different foams. It was shown that differences in the foam properties (b-C12G2 generates a stable foam whereas the foam stabilized by C12E6 is extremely unstable) are due to different surfactant packing at the water/air-interface (structure-property-relation). In the second part dynamic studies of vertical foam films stabilized by b-C12G2, C12E6 as well as by their 1:1 mixture were investigated. With this experiment the lifetime of the foam film, the film thickness, the critical film thickness before rupture and the drainage rate of the vertical foam films were studied. In the last part of the thesis the rheological properties of the water/air-interface, namely the dilational viscoelascticity, were examined. The measured differences in the viscoelasticity of the water/air-interfaces can be correlated with the differences in the foam stabilities. The greater the elasticity the stronger is the resistance against external influences which leads to a more stable foam.